Tutorial: Improving the transient immunity of your microcontroller-based embedded design - Part 1When it comes to protecting their designs from a variety of transient electrical disturbances, developers of microcontroller-based embedded systems in consumer, industrial, and automotive electronics are caught between the rock and the hard place.
On the one hand, more sophisticated and noise-sensitive microcontrollers (MCUs), with high integration, tighter process requirements and lower voltage requirements are moving into designs that are increasingly hazardous with respect to electromagnetic interference (EMI) and electrostatic discharge (ESD).
On the other, increased competition, as well as market regulatory pressures, are forcing original equipment manufacturers (OEMs) to reduce the cost of their products. As a result of this focus on cost control, implementing the necessary transient immunity protections to prevent application malfunction due to transients on power and signal lines is becoming ever more challenging.
The impact of increasing MCU sensitivity and low-cost application design is being felt in all markets: consumer, industrial, automotive, etc. While there are significant differences in the design and use of products for these markets, the susceptibilities induced in all microcontroller-based applications are essentially the same. Typical susceptibilities include unexpected state changes on input pins (reset, interrupt request, or general purpose inputs), corruption of on-chip clock signals, or even damage to the silicon.
The pervasive impact of EMI
and ESD transients
In consumer electronics applications, transient immunity is a challenge for both battery powered and AC mains powered products. Battery powered products such as keyboards, mice, remote keyless entry systems, and remote controls are challenged by ensuring the immunity of the application to ESD transients.
For example, an ESD directly to the product or to a nearby coupling plane has been seen to assert the reset function of an MCU by temporarily changing the state of the RESET pin. In these cases, there is typically a long trace between the RESET pin and an in-circuit programming header that serves as an antenna to receive the ESD energy and couple it to the RESET pin.
Likewise, an electrical fast transient (EFT) injected on the AC power plug of an AC mains powered product can couple to a reset trace by either radiation or conduction resulting in susceptibility. Self-compatibility is also of great concern where the product contains inductive loads (motors, compressors, etc) that are switched.
Since electronics for industrial applications is typically powered only from the AC mains, they have similar issues to those of consumer electronics products powered from the AC mains. However, these issues are typically more severe than in residential applications due to the presence of heavy machinery (large inductive loads) switching on and off the power distribution system for the factory.
The transient immunity issues in automotive electronics applications are also similar to consumer electronics applications except for being powered by DC mains. This is because automobiles contain numerous switching inductive loads (alternator, compressor, solenoids, etc) that put transients on the DC mains. In addition, automotive applications are becoming increasing concerned with ESD.
For example, a new application for MCUs is in remote tire pressure monitoring systems (TPMS). This is a novel innovation but introduces new ESD immunity issues. ESD near a TPMS device has been seen to cause both upset and damage. Since it is almost impossible to ensure that an automotive technician uses and ESD wrist strap, TPMS and other automotive electronics can easily be exposed to ESD in the ±25 kV range during maintenance operations. For reference, commercial electronics are typically required to be immune to ESD in the ±8 kV range.
To meet the transient immunity challenge as traditional power supply designs and EMI controls are sacrificed for lower cost solutions, the embedded systems designers must employ new techniques and processes to meet the applicable regulatory and market requirements for electromagnetic compatibility (EMC).
Achieving transient immunity in a low-cost embedded application can be a difficult and time-consuming process, particularly if not addressed early and often in the design of an application. In addition, making the mistake of not addressing transient protection as close to the AC or DC mains as possible will also adversely affect the complexity of EMC protection. The initial design of an embedded application should maximize EMC so that design budgets and production schedules are met without delays at the EMC compliance stage.
However, there are practical hardware design techniques that can be employed to provide cost-effective protection for EFT, ESD and other power line or signal line transients of short duration. In addition, the EFT or ESD performance of a system can be dramatically affected by the choices made in the software architecture and application functionality.
Software techniques should be viewed as a necessary but last line of defense against adverse system reaction to EFT or ESD events. The software can affect how the system will react to a disturbance if it reaches the MCU but the hardware PCB board and system hardware design should diminish or eliminate the disturbance before it reaches the MCU.
Prioritizing the control of transient voltages and currents with hardware techniques is also important from the aspect of minimizing exposure of electrical and electronic components to over-specification conditions that could result in accelerated failure rates or long-term degradation.
Defining the problem
Low cost, microcontroller-based embedded applications are particularly susceptible to performance degradation during ESD and EFT events. This sensitivity to fast rise time transients is to be expected, even for microcontrollers running at relatively low clock frequencies.
This sensitivity is due to the process technologies employed. Today's semiconductor process technologies for low-cost, 8-bit and 16-bit MCUs implement transistor gate lengths in the 0.65?m to 0.25??m range. These gate lengths are capable of generating and responding to signals with rise times in the sub-nanosecond range (or an equivalent bandwidth of greater than 300MHz). As a result, an MCU is quite capable of responding to ESD or EFT signals injected onto its pins.
In addition to the process technology, MCU performance in the presence of an ESD or EFT event is affected by the design of the IC and its package, the design of the printed circuit board (PCB), the software running on the MCU, the design of the system, and the characteristics of the ESD or EFT waveform when it reaches the MCU. The relative impact of each performance driver (where to focus effort for maximum effect) is shown in the pie chart in Figure 1, below.
|Figure 1. Performance driver impact on transient immunity|
Several facets of IC design other than physical gate length can affect MCU performance during transients. These include the composition of ESD suppression devices on input/output (I/O) pins and the layout of I/O pin structures. ESD devices range from simple diodes to complex active filters. Power supply rejection is accomplished through internal capacitance and careful routing on the die. Physical separation of pin inputs from active circuitry is a proven method to reduce transient effects but at a greater cost penalty due to die size impact.
The choice of MCU package can also affect immunity performance. The package type can have a great influence on PCB layout and composition. Surface-mount MCUs generally have smaller footprints than through-hole packages. This can reduce overall PCB dimensions and increase routing density, but it can also provide more space to implement board-level suppression techniques.
Areas of MCU Vulnerability
Considering that most MCUs are specified and designed to generate and respond to signals with rise times comparable to ESD and EFT events, vulnerability to these events should be expected. Areas of MCUs typically vulnerable to ESD and EFT stresses include:
1) Power and ground pins
2) Edge sensitive digital inputs
3) High frequency digital inputs
4) Analog inputs
5) Clock (oscillator) pins
6) Substrate injection
7) General purpose I/O (GPIO) with multiplexed pin functions
8) ESD protection circuitry
Some MCUs have multiple power and ground pins to isolate high speed digital functions from low speed or noisy analog functions. These supply pins should be filtered appropriately to prevent disturbances in one area from affecting another.
Low cost MCUs may only have a single set of power and ground pins, which makes isolation difficult, and makes filtering more important. It is easy to understand that a transient that gets propagated to a supply line can also disrupt internal circuitry that has no direct route to the pin that was disturbed.
Edge sensitive inputs are particularly vulnerable to transients. These inputs are usually timer or external interrupt inputs. Even with external low pass filtering a sufficiently large pulse can inject enough energy into the input area to disrupt MCU operation. Pulses that don't disrupt the MCU can still be seen as glitches by the MCU. (A software technique to filter out glitches is discussed later in this series).
|Figure 2. Transient generation of logic glitches|
High speed digital inputs, such as clock and data inputs, are less likely to have low pass filtering and consequently can register transients as valid data pulses (see Figure 2, above). External isolation techniques are necessary to eliminate this vulnerability.
Analog inputs are generally lower impedance than digital inputs and can suffer physical damage if not protected during ESD and EFT transients. However, on most MCUs the analog inputs are multiplexed with general purpose I/O pins and have a small sampling window in which the lower input impedance is active. A transient appearing in an analog input pin during an analog to digital conversion will result in distorted data due to the signal disruption. Effective software filtering techniques exist to mitigate this vulnerability.
Most MCUs have a built-in oscillator amplifier so that an external crystal or resonator is all that is needed to ensure a stable high frequency system clock. The oscillator pins can pass noise pulses as valid clock edges and are considered to be the most vulnerable inputs to the system. Appropriate PCB layout is the preferred method to eliminate this risk.
|Figure 3. Transient current injection paths inside the MCU|
As shown in Figure 3 above, transients can travel from the point of entry and affect circuits via several paths. System input signals that exceed the power rails of the MCU will inject current into the I/O pin structure as soon as the signal level exceeds the ESD protection diode's forward voltage.
The I/O pin structure and on chip ESD protection network can dissipate small amounts of injected energy. However, if the injected current is greater than the local circuit can handle, this excessive current can find alternate paths through the supply rails or substrate to disrupt other circuitry. Current injection is generally minimized by using series resistors.
General purpose MCUs have I/O ports that can have more that one function multiplexed on a single pin. An electrical disturbance that causes enough energy to disrupt digital logic can also affect the control circuitry that selects the pin function. The resulting fault could change the pin state, the pin directionality, or the pin function.
Vulnerability is particularly troublesome for general purpose MCUs that are designed to meet the needs of many applications. For these MCUs, it is impractical or impossible to harden all vulnerable areas without adversely affecting functional performance in at least some applications.
Application-specific MCUs can be hardened with greater success, but some vulnerability will continue to exist if the operational frequency or bandwidth of the MCU overlaps the bandwidth of the ESD and EFT signals.
MCU Immunity Performance
The immunity performance for integrated circuits is typically classified into one of four categories as specified in IEC 62132-1 and shown in Table 1 below.
The classification applied is determined by the performance of the integrated circuit in the presence of the disturbance signal (i.e the ESD or EFT waveform). This performance is dependent on the type of integrated circuit and its functional and parametric operation as documented in its data sheet.
|Table 1. Classification of IC EMC Degradation|
Class A performance is the most desirable and is often required for safety-critical applications. Of course, this level of performance is difficult to ensure without taking proper steps in the design of the application. This is because any transient appearing at a pin that can be processed by the input circuitry has the potential for being interpreted as data and corrupting program execution.
Class B performance is considered acceptable for most applications where the main requirement is for no user intervention to recover normal performance. Class C performance can be acceptable for particular applications where operator intervention is not an issue, or where an external watchdog or supervisory circuit is used. Class D performance is not acceptable.
MCU Failure Modes
For MCUs, performance degradation can take many forms. Common forms of temporary degradation include but are not limited to reset, latch-up, memory corruption, and code runaway.
MCUs with internal reset circuits can generally resume operation without operator involvement if the fault is an unexpected reset or code runaway that is caught by a watchdog timer.
Recovery from latch-up and volatile memory (RAM, DRAM, etc.) corruption requires cycling the power to the system. Non-volatile memory (FLASH, EEPROM, ROM, etc.) corruption requires a more extensive process of re-programming the system, which can be viewed as a temporary MCU degradation if the system can be re-worked, or as a permanent degradation if it cannot be re-worked.
Permanent degradation can include increased leakage current on I/O pins which can affect analog measurements, input impedances, and output drive strength. With increased leakage current, the electronic system may still operate within specification for a while, but it may ultimately fail due to damage from the transient stress. Another type of permanent degradation found in transient environments is blown pins due to an electrical overstress.
Impact of MCU Design Trends
The MCU design trend that particularly impacts transient immunity performance is the drive to continually reduce the minimum gate length of individual field effect transistors (FETs), making them smaller and faster. This trend is the result of market pressure on semiconductor manufacturers to reduce the cost of their products by making die sizes smaller.
The result is that maintaining the immunity performance of MCUs in the face of process technology advances is becoming increasingly difficult. When coupled with continuing cost reductions by OEMs at the application or system level, the immunity problem becomes severe.
MCU designers are challenged to develop better methods to dissipate the energy injected during a transient event. While they would appreciate more area in which to include transient suppression circuits, this is generally not allowed in order to keep the die size and cost to a minimum. Some of the remaining options available to the designer include modifying semiconductor attributes (doping and materials) and changing the vertical structure of the I/O pin.
Next in Part 2: Hardware Techniques - The basic circuit building blocks
Ross Carlton has specialized in all aspects of electromagnetic compatibility (EMC) since his graduation from Texas A&M University with a Bachelor of Science in Electrical Engineering in 1985. He has been with Freescale Semiconductor for the last eight years where he has led the EMC design, test and support of Freescale's 8, 16, and 32-bit microcontroller products. In addition, Ross represents the U.S. as a Technical Expert to IEC Subcommittee 47A on integrated circuits where he is the project leader for IEC 61967-2, IEC 61967-3 and IEC 62132-2. He is currently involved in developing transient immunity test methodologies for standardization.
The author would like to thank Greg Racino and John Suchyta, 8-Bit Applications Engineer at Freescale Semiconductor for their inputs and guidance. Their contributions were critical to ensuring consistent and correct guidance.
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